As is well known, magnetoelastic transducers are based on the measurement of the permeability change which takes place in a magnetic material when subjected to the influence of a force. The measuring principles and configuration of these transducers are known and described in a plurality of publications and patent specifications, inter alia in EP 0089916, U.S. Pat. No. 4,506,554 and Swedish patent application 8904307-9. Briefly, it may be stated that a magnetoelastic torque transducer comprises a transducer shaft and a stationary casing comprising a magnetic yoke as well as excitation and measuring windings. However, no detailed description thereof will be given here.
Regardless of which method is used to measure the permeability change, the magnetic hysteresis curve, that is, the B-H curve, traversed by each point in the material during a period of the magnetization, will be changed when the material is, in addition, magnetized by a static field. This, of course, influences the measurement and leads to changes of the measured signal in case of an unloaded transducer, that is the zero signal, and of the sensitivity of the transducer to the influence of forces.
If the transducer is so heavily magnetized by a static field that the ferromagnetic material approaches saturation, the differential permeability as well as the sensitivity of the transducer will be very low. However, the influence remains in connection with considerably lower fields.
To reduce the effect of external static fields, attempts have been made to shield off such fields. However, this has proved to be a difficult technical problem. The traditional method is to surround the transducer with a magnetic shield made of highly permeable material, for example of mu-metal. However, such a design mainly serves as a shield against magnetic fields in the air which are directed across the transducer shaft.
However, according to the above, a torque transducer forms a link in a torque transmission between a driving source and a load. A remanent axial magnetic field in some of the parts in this transmission may then be conducted directly into the transducer shaft without being diverted by an external shield.
The problem with static external magnetization may become particularly serious since static magnetization, by remanence in the transducer material, may also give rise to permanent changes in the operation of the transducer.
The above-mentioned Swedish patent application 8904307-9 discloses an electrical method of minimizing the effect of an external static or quasi-static magnetic field. The method is based on the fact that an external magnetic field causes the measured signal to comprise even harmonics, together with the odd harmonics of the fundamental tone of the magnetization. The magnitude of the even harmonics is a measure of the magnitude of the external magnetic field. By measuring the even harmonics and using this as an input signal to a regulator, the external magnetization may be reduced to zero with the aid of direct current in some suitable winding.